1. Field of the Invention
The present invention relates to acoustical ultrasonic imaging and acoustical holography and, more particularly, to an improved acousto-optic interface enabling photographic recording of the effect of an acoustic wavefront on the interface.
2. Description of the Prior Art
High frequency sound readily travels through comparatively dense homogenous substances, such as liquids or solids. A "test subject," whose internal structure is to be studied (exemplarily, a portion of the human body as described here), may be illuminated by an acoustic wave and the wave modulation studied as a representation of the subject. This subject should be coupled to a sound source, through a liquid medium having an acoustic impedance close to that of the subject.
Water is a convenient form of such a coupling liquid for study of a human subject. When an acoustic wavefront is scattered by somatic tissue and continues, through a water medium, to impact an acousto-optic interface, it can so distort the interface as to form a ripple pattern representing the character of the target tissue. Detection and recording of this distortion can provide a useful representation of the body.
Ultra-sound energy (circa 0.4-20 MHz range) is useful for such diagnostic purposes such as penetrating and imaging soft tissue (especially "in vivo", such as a human foetus in situ, i.e., within the mother's womb), being fluid coupled between an appropriate sound-source transducer and a detection or imaging arrangement. Mechanical scanning systems, commonly known as B-Scan systems are well known to workers in the art and are commonly used in hospitals. These manual systems map a plane of the subject point by point and are slow and require highly trained operators. Newer systems use imaging techniques in which a plane or large volume of the subject are interrogated simultaneously. For instance, in systems like those contemplated by the subject invention, a relatively large, three-dimensional volume (e.g., a cylinder 30 centimeters in diameter by 30 centimeters high), may be imaged with relatively high resolution (on the order of a few millimeters) and using a single, short burst of ultra-sound, on the order of one to several microseconds long.
Such systems can avoid the deleterious effects of subject movement, need no acoustic lenses or scanning devices and apply relatively low average acoustic power. Such systems will be understood by workers in the art to facilitate image generation with greater ease and reliability and not require highly trained technicians. Additionally, a permanent hoolographic film record may be obtained for later viewing and study. Such a hologram also allows a radiologist to focus upon different depths of the test subject. Such a technique is harmless, non-invasive, and non-ionizing, yet is relatively cost-effective and useful for obtaining holographic images of soft tissue structures and organs in human and animal bodies.
A variety of acousto-optic interfaces have been proposed to facilitate recording of scattered acoustic waves. Exemplarily, an oil film on the surface of a water medium might be used; but this is not practical since oil films are very sensitive to vibration and have poor reflectivity. Another interface suggested is a thin plastic film on the surface of a coupling fluid. Thin films are less sensitive to vibration, but their reflecting qualities are nonetheless generally poor. Relatively thicker plastic plates would be conceivable except that they cannot be readily provided with a good optical finish; also, they offer a relatively poor acoustic impedance match with a water medium, causing reflection and a loss of sound energy.
Acousto-optic interfaces heretofore considered in the prior art are, theerefore, not generally satisfactory. Further, when employing acoustical techniques for diagnostic investigation of portions of a human subject it is usually desirable to place the acousto-optic interface below the patient. Obviously, oil films and thin plastic sheets cannot be employed as the interface in such cases, while the thicker plastic sheets will divert too much sonic energy, as well as not accommodating a suitable optical finish.
At (acoustic vibration) frequencies near 1 MHz a large aperture system is needed for medical imaging purposes to provide adequate resolution for reasonably large volumes (e.g., that of the human abdomen). For instance, the system of FIG. 1 would typically use a 30 cm diameter flat acousto-optic interface with a high quality imaging lens. Fabricating a high quality Fourier transform lens of that diameter is a formidable, expensive undertaking. These difficulties are avoided by fashioning a concave-mirror interface according to the invention.